In certain diseased states, the heart lacks sufficient pumping capacity to maintain adequate blood flow to the body's organs and tissues. For example, conditions such as ischemic heart disease and hypertension may leave the heart unable to fill and pump efficiently. This condition, also called congestive heart failure, may lead to serious health complications, including respiratory distress, cardiac asthma, and even death. In fact, congestive heart failure is one of the major causes of death in the Western World.
This inadequacy of the heart can be alleviated by providing a mechanical pump, also referred to as a ventricular assist device (“VAD”), to supplement the pumping action of the heart. VADs may be used to assist the right ventricle, the left ventricle, or both. For example, a VAD may assist the left ventricle by mechanically pumping oxygenated blood from the left ventricle into the aorta.
U.S. Pat. Nos. 9,050,418; 9,173,984; U.S. Pub. No. 2016/0015878 (“the '878 publication) and U.S. application Ser. No. 14/962,511, the disclosures of which are hereby incorporated herein by reference, disclose certain axial flow blood pumps that can be used as ventricular assist devices. When implanted, these and other implantable pumps, typically have an inlet that communicates with a ventricle of a patient and an outlet that communicates with an aorta via an outflow cannula that extends through the ventricle and into the aorta. A pumping element typically resides within the heart and urges oxygenated blood from the ventricle to the aorta.
When implanted and in operation, it is desirable to monitor certain parameters to detect abnormal operating conditions and to determine how the pump affects an environment surrounding the pump. For example, flow information can be obtained directly via an ultrasonic flow sensor or indirectly via a pressure sensor. Such information can be used to provide feedback for control of the pump and to detect a blockage or a situation where the pump outpaces a ventricle's blood supply leading to a suction condition.
Certain existing VADs, such as the centrifugal VAD disclosed in U.S. Pub. No. 2008/0133006, are fitted with an ultrasonic flow sensor for determining parameters associated with blood flow into the VAD's pump. However, such sensors are limited to determining parameters associated with blood flowing into the VAD and are not capable of determining parameters associated with blood flowing out of the VAD and to the remainder of the patient's body. In addition, such sensors are typically located near the pump's impeller and only obtain characteristics of the blood near the impeller rather than at an appreciable distance from the impeller where localized effects are not present. Thus, despite considerable effort devoted to improvements of such VADs, still further improvement are desirable.